The present invention relates generally to joining dissimilar materials and, more particularly, to joining materials having different coefficients of thermal expansion.
The present invention provides a method and a joint for forming an assembly of a first material, for example, a ceramic material, and a second material, for example, a metallic material. The present invention provides consistent joint quality despite high temperature variations to which the joined assembly is subjected and despite the possible significant differences in the coefficients of thermal expansion of the materials constituting the joined assembly.
A more particular object of the invention is to provide a method and joint construction for forming an assembly of a ceramic material, for example, vitreous, fused silica, with a metal material, such as steel.
The method of the invention is preferably used to join at least one metallic piece to a cylinder of vitreous, fused silica. Such a fused silica cylinder, in the form of a roll, is designed to rotatably support a load that is moveably displaced on a series of identical rotatable rolls mounted side by side in a furnace, for example. This load may typically be in the form of a strip or plate of steel or glass material, for example.
U.S. Pat. No. 4,399,598 describes a ceramic roll for transporting sheets of glass during heat treatment thereof. Each cylindrical roll is rotatably mounted on bearings by means of steel ferrules mounted with extra space or "play" provided for thermal expansion at each end of the roll. One or more radially compressible split rings are placed in the space between the outer diameter of the cylindrical roll and the inner diameter of the ferrule.
A roll of a similar type, comprising a cylinder of refractory material, having metal ferrules fixed at the ends, is also known from U.S. Pat. No. 4,404,011. Some space is provided between the ferrules and the ends of the roll. Longitudinal metal strips are placed in these spaces to function as springs to accommodate the play between the ferrules and the roll ends.
These known systems in the prior art make it possible to take up the differential thermal expansions which occur during a change in temperature. In effect, the cylindrical roll of refractory material hardly expands at all, while the metal ferrules expand a great deal. The dimensional play between the roll and the ferrules consequently varies in substantial proportions as a function of the temperature. As a result, it is necessary to provide elastic devices in the spaces between the roll ends and the ferrules to accommodate the play. However, one disadvantage of these prior elastic devices resides in the fact that the joining forces between the roll and the ferrule decrease as the temperature rises. This is due to the drop in the mechanical properties of the elastic springs and also due to the increase in dimensional play resulting from the greater thermal expansion of the metal ferrule. The maximum value of the joining force is obtained during the prestressing of the ferrule on the roll at ambient temperature. This prestressing value is limited by the resistance of the materials. These parameters thus limit the temperature range where good functioning is possible. Assemblies obtained by adhesive bonding are also known in the prior art. However, these assemblies also present shortcomings. The quality of the bond is temperature-limited, caused in part by the loss of mechanical properties of the adhesive at elevated temperatures.
The method and apparatus of the present invention remedies the disadvantages found in the prior art. The invention makes it possible to control the compressive loading in the joint between the metal ferrule and the ceramic roll, for different assembly diameters as well as for different roll and ferrule materials and different operating temperatures.